Printed circuit boards with electrical components mounted thereon are typically coated by a moisture resistant, electrically insulating film. Such films are generally known as conformal coatings, such as silicone, polyurethane, acrylic or epoxy resins. Conformal coatings play an important role in protecting circuit board components from moisture, dust and other contaminants that can adversely affect component performance. Since some components and contacts on a circuit board must remain uncoated, selective coating techniques have been developed. Traditional methods such as standard spray apparatus, brushing and dipping are not suitable for accurate selective application of conformal coatings. Such known systems typically involve a substantial amount of masking and unmasking which is very labor intensive and can lead to extensive rework.
Recently, a new robotically controlled conformal coating application system has been introduced by Nordson Corporation of Westlake, Ohio. This system is sold under the name of the SELECT COAT.RTM. spraying system. This system and components thereof are described in the following U.S. Pat. Nos. 4,753,819; 4,880,663; 5,336,320; and 5,409,733; the entire disclosures of which are fully incorporated herein by reference. In general, a selective coating apparatus such as the SELECT COAT.RTM. spraying system uses an electronic control system to control the operation of a robot-based dispensing unit. The control system controls operation of a robot mechanism for positioning a dispensing head relative to the circuit board and the specific locations thereon where the coating material is to be applied. The control system also controls operation of the dispensing unit to selectively open and close a control valve at the appropriate times to apply coating material to the circuit board. Such systems can be self-contained stand-alone workstations, or alternatively can be integrated into a conveyorized continuous process production line to accommodate high volume operations, including in-line curing ovens.
The dispensing head is an important part of the overall spraying apparatus because the dispensing head shapes the spray pattern of the coating material that is directed to the circuit board. Dispensing options include but are not limited to nonatomizing methods which are typically used with low viscosity coating material (under 100 centipoise, for example) and air assisted atomizing methods for higher viscosity coating materials (for example, 100-3,500 centipoise).
The Nordson SELECT COAT.RTM. spraying system further includes the capability for SWIRL COAT.TM. spray application technology of conformal coating material to a circuit board. The SWIRL COAT.TM. spraying method and apparatus are described in co-pending U.S. patent applications Ser. No. 08/687,790 filed on Jul. 19, 1996, now abandoned, and a continuation-in-part thereof, Ser. No. 08/878,756 filed on Jun. 19, 1997, which applications are both owned in common by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. The referenced applications describe a dispensing head that incorporates a single nozzle design for a plurality of dispensing options. The dispensing options include, but need not be limited to, bead mode dispensing, monofilament mode dispensing and swirl mode dispensing. The apparatus is useful with low viscosity material, high viscosity material and solid materials. Bead mode is generally an airless spray technique, while monofilament and swirl modes utilize an air stream directed at the material stream to impart rotation.
The present invention is directed to improvements in the dispensing head and nozzle configuration for not only the SWIRL COAT.TM. dispensing head used with the SELECT COAT.RTM. spraying system, but for any spray dispensing head that utilizes a twopiece spray nozzle, to improve ease of assembly and maintenance.